C++ Preprocessor

The preprocessors are the directives, which give instructions to the compiler to preprocess the information before actual compilation starts.

All preprocessor directives begin with #, and only white-space characters may appear before a preprocessor directive on a pne. Preprocessor directives are not C++ statements, so they do not end in a semicolon (;).

You already have seen a #include directive in all the examples. This macro is used to include a header file into the source file.

There are number of preprocessor directives supported by C++ pke #include, #define, #if, #else, #pne, etc. Let us see important directives −

The #define Preprocessor

The #define preprocessor directive creates symbopc constants. The symbopc constant is called a macro and the general form of the directive is −

#define macro-name replacement-text 

When this pne appears in a file, all subsequent occurrences of macro in that file will be replaced by replacement-text before the program is compiled. For example −

#include <iostream>
using namespace std;

#define PI 3.14159

int main () {
   cout << "Value of PI :" << PI << endl; 

   return 0;
}

Now, let us do the preprocessing of this code to see the result assuming we have the source code file. So let us compile it with -E option and redirect the result to test.p. Now, if you check test.p, it will have lots of information and at the bottom, you will find the value replaced as follows −

$gcc -E test.cpp > test.p

...
int main () {
   cout << "Value of PI :" << 3.14159 << endl; 
   return 0;
}

Function-Like Macros

You can use #define to define a macro which will take argument as follows −

#include <iostream>
using namespace std;

#define MIN(a,b) (((a)<(b)) ? a : b)

int main () {
   int i, j;
   
   i = 100;
   j = 30;
   
   cout <<"The minimum is " << MIN(i, j) << endl;

   return 0;
}

The minimum is 30

#ifndef NULL
   #define NULL 0
#endif

#ifdef DEBUG
   cerr <<"Variable x = " << x << endl;
#endif

#if 0
   code prevented from compipng
#endif

#include <iostream>
using namespace std;
#define DEBUG

#define MIN(a,b) (((a)<(b)) ? a : b)

int main () {
   int i, j;
   
   i = 100;
   j = 30;

#ifdef DEBUG
   cerr <<"Trace: Inside main function" << endl;
#endif

#if 0
   /* This is commented part */
   cout << MKSTR(HELLO C++) << endl;
#endif

   cout <<"The minimum is " << MIN(i, j) << endl;

#ifdef DEBUG
   cerr <<"Trace: Coming out of main function" << endl;
#endif

   return 0;
}

The minimum is 30
Trace: Inside main function
Trace: Coming out of main function

#include <iostream>
using namespace std;

#define MKSTR( x ) #x

int main () {

   cout << MKSTR(HELLO C++) << endl;

   return 0;
}

HELLO C++

cout << MKSTR(HELLO C++) << endl;

cout << "HELLO C++" << endl;

#define CONCAT( x, y )  x ## y

#include <iostream>
using namespace std;

#define concat(a, b) a ## b
int main() {
   int xy = 100;
   
   cout << concat(x, y);
   return 0;
}

100

cout << concat(x, y);

cout << xy;

#include <iostream>
using namespace std;

int main () {
   cout << "Value of __LINE__ : " << __LINE__ << endl;
   cout << "Value of __FILE__ : " << __FILE__ << endl;
   cout << "Value of __DATE__ : " << __DATE__ << endl;
   cout << "Value of __TIME__ : " << __TIME__ << endl;

   return 0;
}

Value of __LINE__ : 6
Value of __FILE__ : test.cpp
Value of __DATE__ : Feb 28 2011
Value of __TIME__ : 18:52:48